The present invention is directed to wireless communications systems and, more particularly, to methods and apparatus for supporting a plurality of mobile nodes in a communications cell with limited resources.
Wireless communications systems are frequently implemented as one or more communications cells. Each cell normally includes a base station which supports communications with mobile nodes that are located in, or enter, the communications range of the cell""s base station. Within a cell or a sector of a cell, the unit of communications resource is a symbol, e.g., QPSK or QAM transmitted on one frequency tone for one time slot in an orthogonal frequency division multiplexed (OFDM) system. The total available communication resource is divided into a number of such symbols (units) which can be used for communicating control and data information between a base station and one or more mobile nodes in the cell and tends to be limited. Control signals transmitted between a base station and a mobile node may be transmitted in two possible directions, i.e., from the base station to the mobile node or from the mobile node to the base station. Transmission of signals from the base station to the mobile is often called a downlink. In contrast, transmission from the mobile to the base station is commonly referred to as an uplink.
In order to provide efficient use of limited communications resources, base stations may allocate different numbers of tones to different mobile nodes depending on the devices"" bandwidth needs. In a multiple access system, several nodes may be transmitting data, e.g., in the form of symbols, to a base station at the same time using different tones. This is common in OFDM systems. In such systems, it is important that symbols from different mobile nodes arrive at the base station in a synchronized manner, e.g., so the base station can properly determine the symbol period to which a received symbol belongs and signals from different mobile nodes do not interfere with each other. As mobile nodes move in a cell, transmission delay will vary as a function of the distance between a mobile node and a base station. In order to make sure that transmitted symbols will arrive at a base station from different mobile nodes in synchronized manner, timing control signals, e.g., feedback signals, may be and in many cases are, transmitted to each active mobile node of a cellular system. The timing control signals are often specific to each device and represent, e.g., timing corrections of offsets to be used by the device to determine symbol transmission timing. Timing control signaling operations include, e.g., monitoring for timing control signals, decoding received timing control signals, and performing timing control update operations in response to the decoded received timing control signals.
Timing control signals can be particularly important in systems where there are a large number of mobile nodes. In order to avoid interference from a mobile node due to timing miss synchronization, it may be necessary to establish timing synchronization and control before allowing a mobile node to transmit data, e.g., voice data, IP packets including data, etc. to a base station.
In addition to managing limited resources such as bandwidth, power management is often a concern in wireless communications systems. Mobile nodes, e.g., wireless terminals, are often powered by batteries. Since battery power is limited, it is desirable to reduce power requirements and thereby increase the amount of time a mobile node can operate without a battery recharge or battery replacement. In order to minimize power consumption, it is desirable to limit the amount of power used to transmit signals to a base station to the minimal amount of power required. Another advantage of minimizing mobile node transmission power is that it has the additional benefit of limiting the amount of interference that the transmissions will cause in neighboring cells which will often use the same frequencies as an adjoining cell.
In order to facilitate transmission power control, power control signaling, e.g., a feedback loop, may be established between a base station and a mobile node. Power control signaling often takes place at a much faster rate than the timing control signaling. This is because power control signaling attempts to track variations in the signal strength between the base station and the mobile nodes and this can typically vary on the scale of milliseconds. The timing control needs to take into consideration changes in the distance between base station and the mobile nodes and this tends to vary on a much slower scale, typically hundreds of milliseconds to seconds. Thus the amount of control signaling overhead for power control tends to be much more than that for timing control.
In addition to timing and power control signaling, other types of signaling may be employed. For example mobile nodes in addition may also signal on an uplink the quality of the downlink channel. This may be used by the base station to determine the communication resource allocation to allow for the transfer of data packets from the base station to the mobile. Such downlink channel quality reports allows a base station to determine which mobile node to transmit to and if a mobile node is chosen then the amount of forward error correction protection to apply to the data. These downlink channel quality reports generally are signaled on a similar time scale as the power control signaling. As another example, signaling may be used to periodically announce a mobile node""s presence in a cell to a base station. It can also be used to request allocation of uplink resources, e.g., to transmit user data in a communications session. Shared as opposed to dedicated resources may be used for such announcements and/or resource requests.
Signaling resources, e.g., time slots or tones, may be shared or dedicated. In the case of shared time slots or tones, multiple devices may attempt to use the resource, e.g., segment or time slot, to communicate information at the same time. In the case of a shared resource, each ode in the system normally tries to use the resource on an as needed basis. This sometimes results in collisions. In the case of dedicated resources, e.g., with time slots and/or tones being allocated to particular communications device or group of devices to the exclusion of other devices for a certain period of time, the problem of possible collisions is avoided or reduced. The dedicated resources may be part of a common resource, e.g., a common channel, where segments of the channel are dedicated, e.g., allocated, to individual devices or groups of devices where the groups include fewer than the total number of mobile nodes in a cell. For example, in the case of an uplink time segments may be dedicated to individual mobile nodes to prevent the possibility of collisions. In the case of a downlink, time slots may be dedicated to individual devices or, in the case of multicast messages or control signals, to a group of devices which are to receive the same messages and/or control signals. While segments of a common channel may be dedicated to individual nodes at different times, over time multiple nodes will use different segments of the channel thereby making the overall channel common to multiple nodes.
A logical control channel dedicated to an individual mobile node may be comprised of segments of a common channel dedicated for use by the individual mobile node.
Dedicated resources that go unused may be wasteful. However, shared uplink resources which may be accessed by multiple users simultaneously may suffer from a large number of collisions leading to wasted bandwidth and resulting in an unpredictable amount of time required to communicate.
While timing and power control signals and downlink channel quality reports are useful in managing communications in a wireless communications system, due to limited resources it may not be possible for a base station to support a large number of nodes when power control, and other types of signaling need to be supported on a continuous basis for each node in the system.
In view of the above discussion, it is apparent that there is a need for improved methods of allocating limited resources to mobile nodes to permit a relatively large number of nodes to be supported by a single base station with limited communications resources. It is desirable that at least some methods of communications resource allocation and mobile node management take into consideration the need for timing control signaling and the desirability of power control signaling in mobile communications systems.
The present invention is directed to methods and apparatus for supporting multiple wireless terminals, e.g., mobile nodes, using a single base station and limited resources such as bandwidth for the transmission of signals between the base station and mobile nodes, e.g., in a communications cell. A system may be implemented in accordance with the invention as a plurality of cells, each cell including at least one base station which serves multiple mobile nodes. A mobile node can, but need not, move within a cell or between cells.
In accordance with the present invention, mobile nodes support multiple states of operation. The control signaling resources used by a mobile node vary depending on the state of operation. Thus, depending on the state of the mobile node, a large amount of signaling resources may be required while in other states a minimum amount of resources may be required. Control signaling resources are in addition to data transmission resources, e.g., bandwidth used to communicate payload data such as voice, data files, etc. By supporting different mobile node states of operation, requiring differing amounts of base station/mobile node control communications resources, e.g., signal bandwidth, used for control purposes, more mobile nodes can be supported by a base station than could be supported if all mobile nodes were allocated the same amount of communications resources for control signaling purposes.
Bandwidth allocated to a particular mobile device for communicating control signals between the mobile device and a base station is known as dedicated control bandwidth. Dedicated control bandwidth may comprise multiple dedicated logical or physical control channels. In some embodiments, each dedicated control channel corresponds to one or more dedicated segments of a common control channel. Control channel segments may be, e.g., channel time slots used for transmitting and/or receiving control signals. Dedicated uplink control channel segments differ from shared uplink control channel segments where multiple devices share the same bandwidth for uplink signaling.
In the case of a shared communications channel conflicts may result when multiple nodes, at the same time attempt to transmit a control signal using the shared communications channel.
Mobile nodes implemented in accordance with one exemplary embodiment support four states, e.g. modes of operation. The four states are a sleep state, a hold state, an access state, and an on state. Of these the access state is a transitory stage and the other states are steady states and the mobile nodes can be in these states for an extended period of time.
Of the four states, the on state requires the highest amount of control signaling resources, e.g., bandwidth used for control signaling purposes. In this state, the mobile node is allocated bandwidth on as needed basis for transmitting and receiving traffic data, e.g., payload information such as text or video. Thus, at any given time in the on state a mobile node may be allocated a dedicated data channel for transmitting payload information. In the on state the mobile node is also allocated a dedicated uplink control signaling channel.
In various embodiments, a dedicated uplink control channel is used during the on state by the MN to make downlink channel quality reports, communicate resource requests, implement session signaling, etc. Downlink channel quality reports are normally signaled frequently enough to track variations in the signal strength between the base station and the mobile node.
During the on state, the base station and mobile node exchange timing control signals using one or more dedicated control channels allowing the mobile node to periodically adjust its transmission timing, e.g., symbol timing, to take into consideration changes in distance and other factors which might cause the transmitted signals to drift timing from the base station""s perspective, with the signals transmitted by other mobile nodes. As discussed above, the use of timing control signaling and performing timing control signaling operations, such as updating transmission timing, is important in many systems which use orthogonal frequency division multiple access in the uplink to avoid interference from transmission signals generated by multiple nodes in the same cell.
To provide transmission power control, during the on state, transmission power control signaling is employed to provide a feedback mechanism whereby a mobile node is able to efficiently control its transmission power levels based on signals periodically received from the base station with which it is communicating. In various embodiments the base station periodically transmits power control signals over a dedicated control downlink. As part of the transmission power control signaling process, the mobile node, performs various transmission power control signaling operations including, for example, monitoring for transmission power control signals directed to the particular mobile node, decoding received transmission power control signals, and updating its transmission power levels based on the received and decoded transmission power control signals. Thus, in response to receiving power control signals in a dedicated downlink segment corresponding to the particular mobile node, the mobile node adjusts its transmission power level in response to the received signal. In this manner, a mobile node can increase or decease its transmission power to provide for successful receipt of signals by the base station without excessive wastage of power and therefore reducing interference and improving battery life. The power control signaling is typically carried out sufficiently frequently to track fast variations in the signal strength between the base station and the mobile nodes. The power control interval is a function of smallest channel coherence time that the system is designed for. The power control signaling and the downlink channel quality reports are normally of similar time scale, and in general, occur at a much higher frequency than the timing control signaling. However, in accordance with one feature of the present invention the base station varies the rate at which it transmits power control signals to a mobile node as a function of the mobile node""s state of operation. As a result, in such an embodiment, the rate at which the mobile node performs transmission power control adjustments will vary as a function of the state in which the mobile node operates. In one exemplary embodiment, power control updates are not performed in the sleep state and, when performed in the hold state, are normally performed at a lower rate than during the on state.
Operation of a mobile node in the hold state requires fewer control communications resources, e.g., bandwidth, than are required to support operation of a mobile node in the on state. In addition, in various embodiments while in the hold state a mobile node is denied bandwidth for transmitting payload data, but the mobile can be allocated bandwidth for receiving payload data. In such embodiments the mobile node is denied a dedicated data uplink communications channel during the hold state. The bandwidth allocated for receiving data may be, e.g., a data downlink channel shared with other mobile nodes. During the hold state timing control signaling is maintained and the mobile node is also allocated a dedicated control uplink communication resource, e.g., dedicated uplink control communications channel, which it can use to request changes to other states. This allows, for example, a mobile node to obtain additional communications resources by requesting a transition to the on state where it could transmit payload data. In some but not all embodiments, in the hold state, the dedicated uplink control channel is limited to the communication of signals requesting permission to change the state of mobile node operation, e.g., from the hold state to the on state. During the hold state the bandwidth allocated, e.g., dedicated, to a mobile node for control signaling purposes is less than in the on-state.
Maintaining timing control while in the hold-state allows the mobile nodes to transmit their uplink requests without generating interference to other mobiles within the same cell and having a dedicated uplink control resource ensures that the delays for state transition are minimal as the requests for state transitions do not collide with similar requests from other mobile nodes as may occur in the case of shared uplink resources. Since timing control signaling is maintained, when the mobile node transitions from the hold state to the on state it can transmit data without much delay, e.g., as soon as the requested uplink resource is granted, without concerns about creating interference for other mobile nodes in the cell due to drift of uplink symbol timing. During the hold state, transmission power control signaling may be discontinued or performed less frequently, e.g., at greater intervals than performed during on state operation. In this manner, the dedicated control resources used for power control signaling can be eliminated or reduced allowing fewer resources to be dedicated to this purpose than would be possible if power control signaling for all nodes in the hold state was performed at the same rate as in the on state.
When transitioning from the hold state to the on state, the mobile node may start off with an initial high power level to insure that its signals are received by the base station with the power level being reduced once transmission power control signaling resumes at a normal rate as part of on state operation. In one exemplary embodiment, when the mobile node in the hold state intends to migrate to the on state, it transmits a state transition request using a dedicated uplink communication resource, which is not shared with any other mobile nodes. The base station then responds with a broadcast message indicating its response to the mobile""s state transition request. The mobile on receiving the base station message meant for it responds with an acknowledgement. The acknowledgment is transmitted over a shared resource on the uplink and is slaved to the broadcast message on the downlink.
By transmitting an appropriate state transition request the mobile may also transition to the sleep state. In one exemplary embodiment, when the mobile node does not intend to migrate to another state, the mobile node may not transmit any signal in its dedicated uplink communication channel, though the dedicated channel has been assigned to the mobile node and is therefore not used by any other mobile nodes. In another embodiment, the mobile node uses an on/off signaling in its dedicated uplink communication channel, where the mobile node sends a fixed signal (on) when it intends to migrate to another state and does not send any signal (off) when it does not intend to migrate to any other state. In this case, the transmission of the fixed signal can be interpreted as a migration request to the on state if the transmission occurs at certain time instances, and as a migration request to the sleep state if the transmission occurs at some other time instances.
In order to support a large number of mobile nodes, a sleep state requiring relatively few communications resources is also supported. In an exemplary embodiment, during the sleep state, timing control signal and power control signaling are not supported.
Thus, in the sleep state, the mobile nodes normally do not performing transmission timing control or transmission power control signaling operations such as receiving, decoding and using timing and transmission power control signals. In addition, the mobile node is not allocated a dedicated uplink control resource, e.g., uplink control communications channel, for making state transition requests or payload transmission requests. In addition, during the sleep state the mobile node is not allocated data transmission resources, e.g., dedicated bandwidth, for use in transmitting payload data, e.g., as part of a communications session with another node conducted through the base station.
Given the absence of a dedicated uplink control channel during the sleep state, a shared communications channel is used to contact the base station to request resources necessary for a mobile node to initiate transition from the sleep state to another state.
In some embodiments, in the sleep state the mobile node may, at the behest of the base station serving the cell, signal its presence in the cell, e.g., using a shared communications resource. However, as discussed above, little other signaling is supported during this state of operation. Thus, very little control signaling bandwidth is used to communicate control information between mobile nodes in the sleep state and a base station serving the nodes.
The access state is a state through which a node in the sleep state can transition into one of the other supported states. The transition between states may be triggered by an action by a user of the mobile node, e.g., an attempt to transmit data to another mobile node. Upon entering the access state, transmission power control and timing control signaling has not yet been established. During access state operation, timing control signaling is established and, in various embodiments, full or partial transmission power control signaling is established. A mobile node can transition from the access state to either the on state or the hold state.
The establishment of the timing synchronization and transmission power control can take some amount of time during which data transition is delayed. Also the access process happens through a shared media and contentions between mobile nodes need to be resolved. By supporting a hold state in accordance with the present invention, in addition to a sleep state, such delays can be avoided for a number of mobile nodes, as transition from the hold state to the on state does not go through the access state, while the number of nodes which can be supported by a single base station is larger than would be possible without the use of reduced signaling states of mobile node operation.
In some embodiments, for an individual cell, the maximum number of mobile nodes that can be in the sleep state at any given time is set to be greater than the maximum number of mobile nodes that can be in the hold state at given time. In addition, the maximum number of mobile nodes which can be in the hold at any given time is set to be greater than the maximum number of nodes that can be in the on state at any given time.
In accordance with a power conservation feature of the present invention, downlink control signaling from the base station to the mobile nodes is divided into a plurality of control channels. A different number of downlink control channels are monitored by a mobile node depending on the node""s state of operation. During the on state the greatest number of downlink control channels are monitored. During the hold state a smaller number of downlink control channels are monitored than during the on state. In the sleep state the smallest number of downlink control channels are monitored.
To further reduce power consumption in the mobile node associated with monitoring for control signals, in accordance with one feature of the invention control channels monitored during the hold and sleep states are implemented as periodic control channels. That is, signals are not broadcast on a continuous basis on the control channels monitored in the hold and sleep states. Thus, during the hold and sleep states the mobiles monitor for control signals at periodic intervals and save power by not monitoring for control signals at those times when control signals are not transmitted on the monitored channels. To further decrease the time a particular mobile needs to monitor for control signals during the hold and sleep states, portions, e.g., segments, of the periodic control channels may be dedicated to one or a group of mobile nodes. The mobile nodes are made aware of which control channel segments are dedicated to them and then monitor the dedicated segments as opposed to all the segments in the control channels. This allows monitoring for control signals to be performed in the hold and sleep states by individual mobile nodes at greater periodic intervals than would be possible if the mobile were required to monitor all segments of the periodic control channels.
In one particular embodiment, during the on state, mobile nodes monitor segments of an assignment channel on a continuous basis and also monitor segments of periodic fast paging and slow paging control channels. When in the hold state the mobiles monitor the fast paging and slow paging control channels. Such monitoring may involve monitoring a subset of the segments of the periodic fast and slow paging channels, e.g., segments dedicated to the particular mobile node. During the hold state in the particular exemplary embodiment the slow paging channel is monitored but not the fast paging channel or the assignment channel. The paging control channels may be used to instruct the mobile node to change states.
By limiting the number of control channels and the rate of control channel monitoring as a function of the state of operation, power resources can be conserved in accordance with the invention while operating in the hold and sleep states.
Numerous additional features, benefits and details of the methods and apparatus of the present invention are described in the detailed description which follows.